Spiral Balance tip color choice

A quick learner from Winchester, NH says:
I need to order series 600-21 spiral balances 5/8" by 21" long, but I am confused. It is for a lower sash that weighs 21 lbs. The original factory "Alside" started shipping me red tip balances and then they started shipping me blue tip balances. Your website says that a red tip balance is for 6 to 19 lbs. and a blue tip balance is for 10 to 29 lbs.; however another website says that a red tip balance is for 13 to 25 lbs. and a blue tip is for 26 to 35 lbs. Should I order a blue tip or red tip balance?
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Tom from SWISCO responded:
I cannot speak for the other site or how they calculate their spring strength, but if you have a 21 pound sash then I would recommend that you use the Blue Tip color option for our S600-21. The Red Tip will not be strong enough.
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